The present invention relates to horizontal synchronizing oscillation circuits of a display, and more particularly to a horizontal synchronizing oscillation circuit to be used in a display which can reproduce correct images even when image signals having different horizontal scan frequencies are inputted.
In a display corresponding to a plurality of horizontal scan frequencies as shown in the prior art, as disclosed in K. Adachi, et al. Japanese patent publication No. 61-8628 (1986) "Synchronized Oscillator", Published Mar. 15, 1986, DC voltage proportional to frequency of the horizontal synchronizing signal is generated and supplied to a control voltage terminal of a voltage controlled oscillator VCO of a horizontal AFC (automatic frequency control) circuit, thereby free-run frequency of the VCO is controlled and the oscillation output signal having frequency corresponding to the horizontal scan frequency of the input image signal can be obtained.
FIG. 1 shows a horizontal synchronizing oscillation circuit of a display corresponding to a plurality of horizontal scan frequencies in the prior art.
The horizontal synchronizing oscillation circuit in the prior art comprises a terminal 1 to which horizontal synchronizing signal is inputted; an F/V conversion circuit which is connected to the terminal 1 and receives the inputted synchronizing signal and generates voltage corresponding to its frequency; a power source 3 which is connected to the F/V conversion circuit 2 and varies the power source voltage to be supplied to a power source voltage terminal of a horizontal output circuit 4 in response to a signal from the F/V conversion circuit 2, i.e., in response to the frequency of the synchronizing signal; a voltage controlled oscillator (VCO) 6 which supplies drive pulses to a horizontal output transistor 41 of the horizontal output circuit 4; and a phase comparator 5 which receives the synchronizing signal from the terminal 1 and sawtooth wave signal of horizontal period of the horizontal output circuit 4, and detects phase difference between both input signals and supplies the detected signal to a voltage control terminal of the voltage controlled oscillator (VCO) 6.
The horizontal output circuit 4, the phase comparator 5 and the voltage controlled oscillator (VCO) 6 constitute a horizontal AFC circuit.
Output of the power source 3 is applied as power source voltage to the horizontal output circuit 4, and supplied to the voltage control terminal of the voltage controlled oscillator (VCO) 6. In the voltage controlled oscillator (VCO) 6, the power source voltage from the power source 3 and the control voltage from the phase comparator 5 are combined, and frequency of the generated signal is varied in response to the combined control signal.
In the horizontal output circuit 4, peak value .sup.I DYpp of the horizontal deflection current flowing in a horizontal deflection coil DY is expressed by following formula. ##EQU1## LDY : inductance of horizontal deflection coil TH : horizontal period
Tr : horizontal flyback period PA1 EfH : power source voltage of horizontal output circuit 4
From formula (1) it is understood that, in order to have the deflection current flow remain constant throughout the wide horizontal frequency in the horizontal deflection coil DY, the power source voltage EfH must be varied in proportion to the frequency fH (=1/TH) of the horizontal synchronizing signal.
In the circuit of FIG. 1, the frequency of the horizontal synchronizing signal is converted into voltage by the F/V conversion circuit 2, and the power source voltage to be supplied to the power source voltage terminal of the horizontal output circuit 4 is varied in the power source 3 in response to the voltage signal, i.e., in response to the frequency of the horizontal synchronizing signal. In other words, if the frequency of the horizontal synchronizing signal becomes higher, the output of the F/V conversion circuit 2 is increased and the power source voltage .sup.E fH is increased. Further, the power source voltage .sup.E fH which varies corresponding to the frequency of the horizontal synchronizing signal is supplied to the voltage control terminal of the voltage controlled oscillator (VCO) 6, and free-run frequency of the voltage controlled oscillator (VCO) 6 is varied, thereby the horizontal AFC circuit is synchronized with the horizontal synchronizing signal in the wide horizontal deflection frequency range.
In the horizontal oscillation circuit of the prior art, however, when the power source is turned on or off, or when the horizontal scan frequency is changed, the output transistor 41 may be damaged. The damage may occur if the increase of the oscillation frequency f.sub.H, of the VCO 6 cannot follow the rapid increase of the power source voltage E.sub.fH, and the decrease of the effective scan period .DELTA.(T.sub.H - T.sub.r).apprxeq.1/.DELTA.f.sub.H ' in the horizontal direction becomes extremely small in comparison to the increase of the variation .DELTA.E.sub.fH of the power source voltage E.sub.fH in formula (1). This causes the deflection current I.sub.DY, determined in proportion to product of (T.sub.H -Tr), and the power source voltage EfH, to become abnormally large and as a result the output transistor 41 may be damaged.